The present invention relates to a radio frequency (RF) power amplifier for amplifying an RF signal with a high power efficiency and with a minimum of distortions by use of a GaAs field effect transistor (FET) and, more particularly, to an RF power amplifier feasible for the power amplification of an ON/OFF RF signal, e.g., TDMA (Time Division Multiple Access) signal to be sent from a digital portable telephone.
It has been customary with the above-described type of RF power amplifier to use amplification, i.e., class "AB" or class "B" amplification for the purpose of enhancing the radio of RF signal output to supplied DC power, i.e., a power efficiency. Specifically, with such an RF power amplifier, it is a common practice to reduce the drain current, or idling current, of the FET to less than about one-tenth of saturation drain current (class "AB" amplification) when an RF signal is not inputted or when a small RF signal is inputted. When an RF signal of regular level is inputted to the RF power amplifier (generally during high amplitude operation), the drain current of the FET sharply increases even to a level four or five times higher than the idling current. Nevertheless, a high power efficiency is achievable since the gain and RF signal output in such a condition increase more than the drain current.
However, the problem with the RF power amplifier is that the nonlinear amplification mentioned above noticeably distorts the RF signal output due to, among others, intermodulations (particularly odd intermodulations, e.g., IM3, IM5 and IM7). For example, when a digital signal modulated by QPSK (Quadrature Phase-Shift Keying) is applied to the RF power amplifier, the distortion of the RF signal output is, when evaluated in terms of nearby channel leak power, found to increase beyond a limit regarding the specifications of a system using the QPSK modulated wave.
In this connection, one of conventional RF power amplifiers of the type performing class "AB" amplification with an FET uses a module FM C090902 (manufactured by Fujitsu Ltd., Kawasaki, Japan) and applied a voltage of 5.8 V to between the drain and the source of the FET to thereby cause an idling current of 150 mA to flow. When an RF signal of 950 MHz and +3 dBm is inputted to such a power amplifier, an RF signal output of +32 dBm (gain of 29 dB) and an FET drain current of 600 mA are obtained, insuring a satisfactory power efficiency. However, the 50 kHz nearby channel leak power of this module is as great as 35 dB in terms of RF signal output ratio.